Abstract
Sintered porous silicon is a well-known seed for homo-epitaxy that enables fabricating transferrable monocrystalline foils. The crystalline quality of these foils depends on the surface roughness and the strain of this porous seed, which should both be minimized. In order to provide guidelines for an optimum foil growth, we present a systematic investigation of the impact of the thickness of this seed and of its sintering time prior to epitaxial growth on strain and surface roughness. Strain and surface roughness were monitored in monolayers and double layers with different porosities as a function of seed thickness and of sintering time by high-resolution X-ray diffraction and profilometry, respectively. Unexpectedly, we found that strain in double and monolayers evolves in opposite ways with respect to layer thickness. This suggests that an interaction between layers in multiple stacks is to be considered. We also found that if higher seed thickness and longer annealing time are to be preferred to minimize the strain in double layers, the opposite is required to achieve smoother layers. The impact of these two parameters may be explained by considering the morphological evolution of the pores upon sintering and, in particular, the disappearance of interconnections between the porous seed and the bulk as well as the enlargement of pores near the surface. An optimum epitaxial growth hence calls for a trade-off in seed thickness and annealing time, between minimum-strained layers and rougher surfaces.PACS codes81.40.-z Treatment of materials and its effects on microstructure, nanostructure, and properties; 81.05.Rm Porous materials; granular materials; 82.80.Ej X-ray, Mössbauer and other γ-ray spectroscopic analysis methods
Highlights
Nowadays, about 30% of the cost of a wafer-based silicon solar cell is due to the silicon material itself
The case of porous silicon (PSi) monolayers To investigate the effect of the thickness of the PSi stack, on the strain and surface roughness, several PSi layers were prepared with different thicknesses and porosities as summarized in Table 1
In this work, we studied the impact of two factors on the quality of highly boron doped PSi double layers as epitaxy seed layers: strain and surface roughness
Summary
About 30% of the cost of a wafer-based silicon solar cell is due to the silicon material itself. Researchers are aiming at reducing the consumption of silicon while keeping the cell efficiency high One of these attempts is employing a layer-transfer process (LTP) where an active silicon layer is epitaxially grown using chemical vapor deposition (CVD) on porous silicon (PSi), which acts as the detachment layer and as the epitaxy-seed layer [1,2]. Transferring the epitaxial layer (silicon “epi-foils”) to foreign low-cost substrates, while the parent substrate can be reused, would allow for cost-effective solar cells The intrinsic stress present in the PSi layer causes strain during epitaxial growth or even during the cool down [3]
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